The present invention relates to the art of acoustical measuring and gaging, particularly gaging the relative position of acoustically coupled objects as they undergo relative movement. The present invention finds particular application in gaging the relative position of an acoustical defect detection assembly and an elongated examined object, such as a metal pipe or tube, and will be described with particular reference thereto. It is to be appreciated, however, that the invention also finds utility in gaging the position of other acoustically coupled objects, such as a workpiece and an assembly for performing machining or drilling operations, a workpiece and an optical examination assembly, a robotic manipulation assembly and a guide track therefor, and the like.
Metal pipe and tubing of the type used in oil well drilling is commonly manufactured such that lengths in excess of 50 feet are routine. In well drilling, nuclear reactors, and other demanding installations, the tubular goods are commonly inspected at the point of use. Small defects, such as hairline cracks, can cause premature failure in an area which is not readily accessible for repair. These debilitating cracks commonly have dimensions as small as 10 mils (0.010 inches) or so. To locate this type of defect, the pipes may be inspected by means of an acoustical defect detection system. When the detection system locates a defect, the location of the defect must also be determined.
To be sure that defects or cracks of all orientations are located, it is common to examine the tubing a plurality of times, wherein a different mode of examination is employed each time. In order to determine whether two modes of examination are viewing the same defect or two closely adjacent defects, it is desirable to locate each defect with at least as fine a precision as the dimension of the flaw.
Heretofore, the relative position of the ultrasonic examination system and the workpiece was determined with a mechanical measurement system. These systems determine relative position by mechanically measuring various distances such as displacement of a cable for pulling a workpiece holder, rotational position of a rotating screw drive for the workpiece holder, rotation of workpiece supporting or engaging rollers, and the like. However, a measurement on the order of even 100 mils over a length of 50 feet would require precision on the order of a hundredth of a percent. Mechanical systems capable of measuring such long distances with this precision are unsuited to field installations.
The present invention contemplates new and improved method and apparatus which overcome the foregoing problems and others, and which facilitate measuring distances along a large or elongated object with high precision.